Such a kernel is called   bubble [1]}, and can be pictured graphically as a 6-colored bipartite regular graph, with black and white vertices corresponding respectively to the fields \(T\)  and \(\bar{T}\) , and each line corresponding to a Kronecker delta. As an example, the 4-valent bubble associated to the kernel \(\mathcal {W}^{(1)}\)  is depicted on Figure (REF ) below, and in the same way, all the interaction bubbles involved in the action \(S_{int}\)  are defined as:
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